Dynamic Variation and Correlation of Pericarp Tenderness and Component Contents of Super Sweet Corn (sh2) in Kernel Development

Shilong ZHANG, Xiaoqin LI, Zhenghua HE, Yiqin HUANG

Abstract Data on the mechanism of how pericarp components in fresh sweet corn affect pericarp tenderness are scarce. This study explored variation and correlation of pericarp tenderness and components over time in three inbred lines of sweet corn that were selected for their differences in pericarp tenderness. The three lines presented a curvilinear increase in pericarp tenderness （puncture reading） from 12 to 24 d after pollination （DAP） across two environments， with the means at each time point always in the same order： S33205>T105>PE10. Pericarp tenderness difference at each same time point in two environments over time varied similarly in each of the three inbred lines： increasing first， then dropping after peak， and insignificant at the end. Of the main pericarp components， mean contents were in the order： hemicellulose>cellulose>lignin in both environments at each time point for the three inbred lines. As the pericarp developed， hemicellulose content increased gradually， lignin content varied along a single-peak curve， cellulose content fluctuated around 24%， and pectin and ash contents changed slightly and irregularly. The pericarp of PE10， with the most tender pericarp， accumulated hemicellulose faster than other two lines. Hemicellulose and lignin contents were significantly correlated with pericarp tenderness， and the main components affected pericarp tenderness.

Key words Super sweet corn; Pericarp tenderness; Main ingredients; Growth curve; Interrelation

Received： April 3， 2022  Accepted： June 5， 2022

Supported by The Open Project of the State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources （SKL-CUSAb-2013-03）.

Shilong ZHANG （1973-）， male， P. R. China， associate researcher， devoted to research about genetic breeding of maize.

*Corresponding author. E-mail： hbmaize@163.com.

Currently， shrunken sweet corn （sh2） is widely used for fresh and processed corn. Mutants with high sugar and moisture contents can be retained longer after harvest than most other mutants of homozygous hybrids for sh2 mutation[1]. In fresh sweet corn， the primary eating quality concerns of consumers are sweetness and kernel tenderness[2]. Selection for tender， crispy genotypes with high sugar and low starch concentrations and enhanced aroma may increase eating quality of sweet corn. Many research groups have studied the eating quality of maize[3-5]. Tenderness is a key sensory attribute that determines the overall acceptability of fresh sweet corn[6-7]; and pericarp tenderness is the predisposition of the pericarp to fragmentation by chewing[8]. Early studies reported negative correlations between pericarp thickness and pericarp tenderness and the sensory perception of tenderness[4，9-11]. Therefore， pericarp thickness， or the outer layer of the maize kernel， is an important factor for improving the eating quality of sweet corn， and a major selection target for improving sweet corn tenderness[4，12] and the popping ability of popcorn[13]. A corn strain with a thin pericarp is the preferred edible variety[4，9，14-15]. However， recent investigations found no significant correlation between pericarp thickness and eating quality score. Thus， pericarp thickness may not be the main factor affecting the eating quality fresh corn[16]. The mechanical properties of the pericarp from sweet corn inbred lines are important factors of pericarp tenderness， as well as the eating quality of fresh corn[17].

The cell wall is a key structural component of plants and many plant-derived foods. For decades， the cell wall has been known to play a central role in determining the quality characteristics of many plant-based foods， particularly texture[18]. Plant cell walls are composed primarily of cellulose， hemicelluloses， lignin， and pectic polysaccharides with minor structural proteins[19]. Cellulose is one of the most abundant biopolymers in nature[20]， with a straight carbohydrate polymer chain comprising β-1，4-glucans[21-22]. Hemicelluloses are a class of heterogeneous polysaccharides， and lignin is a stable and complex waterproofing phenolic polymer comprising p-coumaryl alcohol （H）， coniferyl alcohol （G）， and sinapyl alcohol （S）[23-26]. Lignin contributes to lignocellulosic recalcitrance due to its structural diversity and heterogeneity[27-28].

This study investigated the role of the main components of the pericarp cell wall in super sweet corn （sh2） to identify chemical factors affecting pericarp tenderness. We investigated ① variation in the tenderness and main components of the pericarp during kernel development and ② the correlation between pericarp tenderness and the main components of the pericarp in super sweet corn （sh2）.

Materials and Methods

Plant materials and experiment design

Ten inbred lines of super sweet corn—PE10， S13237， T105， S33222， S33205， HZ508， S13084， S23207， S33247， and S23288—with similar growth periods but differences in pericarp tenderness were used in this study. The lines were provided by Hubei Key Laboratory of Food Crop Germplasms and Genetic Improvement/Food Crop Institute， Hubei Academy of Agricultural Science. A preliminary evaluation of the ten inbred lines revealed that PE10 had the best pericarp tenderness， T105 ranked middle， and S33205 had the toughest pericarp， which were selected for the remainder of the study. A randomized block design was used with three replicates， each comprising ten 6-m long rows spaced 0.67 m apart. The plants were sown 0.33 m apart. Recommended herbicide and pesticide regimens for field practice were followed.

Plant sample collection

Seeds were sown at the experimental base of the Institute of Food Crops， Hubei Academy of Agriculture Sciences， Wuhan， Hubei Province on March 25， 2014. Flowering occurred in early June. Each ear was bagged before silking. Most plants self-pollinated on the same day， which was recorded. In November 2014， the field trial was repeated in Timeng Village， Lingshui County， Hainan Province.

Sampling occurred at 12， 14， 16， 18， 20， 22， and 24 d after pollination （DAP） for PE10， T105 and S33205. The remaining lines were only sampled on the optimum picking day at each site： 18 DAP in Wuhan and 22 DAP in Lingshui according to the preliminary evaluation. Ten uniform ears per line were randomly harvested from each plot per sampling.

Raw pericarp tenderness test

Immediately after harvesting， the ears were transported in ice boxes to a laboratory. Three uniform peeled cobs from each plot were evaluated for pericarp tenderness. Pericarp tenderness was measured as the force puncturing the pericarp through a kernel arc crown center perpendicular to the tangent line for the center point with an Accuforce Cadet Force gauge （model #ML4432-3; Ametek， Largo， Fla.） with a 1.74-mm circular probe， as described by Teri et al.[17] with a slight revision. Twelve kernels in the center portion of a cob were measured， and the mean of 10 readings （excluding maximum and minimum） was divided by the cross-sectional area of the rod to obtain standardized measurements （g/mm2）. The average of three cobs was recorded as the pericarp tenderness of inbred line， and the bigger the value， the poorer the pericarp tenderness.

After determining raw pericarp tenderness， the ears were sealed in freezer bags and stored in a -70 ℃ freezer for later evaluation of the chemical components of the pericarp. The crown portion of each kernel located on the center of each frozen ear was removed with a razor blade and peeled off with tweezers. The remaining pericarps containing the aleurone layer or loose inner pericarp were scraped off with a thumbnail to ensure semitransparency. After being washed with deionized water， pericarp samples were freeze-dried， ground into powder through a 40-mesh sieve with a coffee grinder， and stored in a dry container until use.

Wall polymer fractionation

Plant cell wall fractionation was conducted according to Peng et al.[29]， with minor modifications according to Huang et al.[30]. All experiments were carried out in biological triplicate.

Colorimetric determination of hexoses， pentoses， and uronic acids

A UV-VIS spectrometer （V-1100D， Shanghai MAPADA Instruments Co.， Ltd.， Shanghai， China） was used for hexose， pentose， and uronic acid assays， as described by Huang et al.[31]. Hexoses were detected by anthrone/H2SO4[32]， pentoses by orcinol/HCl[33]， and total uronic acids by m-hydroxybiphenyl/NaOH[32]. High pentose levels affected absorbance readings at 620 nm for hexose assay. Thus， a deduction from pentoses was conducted for final hexose calculation using a series of xylose concentrations to plot the standard curve referred for the deduction， which was verified by GC-MS analysis. All experiments were conducted in biological triplicate.

Detection of three wall polymers and total ash content

Cellulose was measured by hexose assay， and hemicelluloses were detected by calculating the total hexoses and pentoses of the hemicellulose fraction. Lignin was determined by two-step acid hydrolysis according to the Laboratory Analytical Procedure of the National Renewable Energy Laboratory[34]. The total ash content assay was conducted as described by Sluiter et al.[34].

Detection of pectin

For pectin detection， 0.100 0 g （precise to four decimals） of dry sample was weighed， and soluble saccharides， lipid， and starch removed after fractionation. Pellets were treated with 5.0 ml of 0.5% （w/v） ammonium oxalate （AO） for 1 h at 100 ℃， and shaken every 10 min to avoid the accumulation of insoluble substances on the solution surface. The mixture was centrifuged at 3 000 g for 5 min to collect the supernatant. The remaining residue was washed once with 5.0 ml of 0.5% （w/v） AO and twice with distilled water. The supernatant was collected to meter volume. Pentose， hexose， and uronic acid contents were determined by colorimetric method. Pectin content is the sum of pentoses， pexoses， and uronic acid.

Statistical analyses

Data were presented as the mean±SD （n=3） and subjected to analysis of variance by SAS General Linear Model procedures according to a randomized block design[35]. When F values were significant， least significant difference multiple comparisons tests （LSD-tests） were used to identify significant pairwise differences between means. Relationships between parameters were evaluated by calculating the coefficients of correlation. P<0.01 and P<0.05 were considered statistically significant at 1% and 5% for all tests， respectively.

Results

Pericarp tenderness in 10 inbred lines of super sweet corn picked at the optimum picking time

Two-way analysis of variance was conducted on the pericarp tenderness of 10 inbred lines of super sweet corn picked at the optimum picking time in two environments （Table 1）.

Pericarp tenderness significantly differed between the 10 inbred lines. Pericarp tenderness of each inbred line picked at the optimum picking time did not significantly differ between environments， and there was no significant inbred line×environment interaction. Of the 10 inbred lines， PE10 had best pericarp tenderness and S33205 had the poorest. No significant differences in pericarp tenderness were found between HZ508 and S13084， S13084 and S23207， or S23207 and T105. Differences between other lines were significant or highly significant （Table 2）.

Pericarp tenderness during kernel development

From 12-24 DAP， variations in pericarp tenderness （puncture reading） were similar across both environments for PE10， T105， and S33205 （Fig. 1）. Generally， pericarp tenderness increased with increasing DAP （Figs. 1A， B）. The mean pericarp tenderness of the three inbred lines at each sampling time were ranked S33205>T105>PE10， regardless of the environment. The mean ranges of pericarp tenderness were all ranked PE10>S33205>T105 in two environments from 12-24 DAP （Fig. 1）， and the difference in the mean ranges of pericarp tenderness between two environments was small for the same inbred line from 12-24 DAP （Fig. 2）.

However， pericarp tenderness varied within the three inbred lines between environments， despite similar trends （Fig. 2）. At 12 DAP， pericarp tenderness did not significantly differ between environments， but as the kernels developed， differences gradually increased from 16 DAP， before becoming insignificant at the final harvest. The average pericarp tenderness in spring 2014 （Wuhan， Hubei） was greater than that in winter （Lingshui， Hainan） at each same sampling time， but similar at the optimum picking times in two environments. That is， pericarp tenderness at 18 DAP （optimum picking time） in spring 2014 was similar to that at 22 DAP （optimum picking time） in winter 2014.

Variation in the main components of the pericarp over time in super sweet corn

With kernel development， the hemicellulose content of the pericarp in the three inbred lines gradually increased， the lignin content varied along a single-peak curve with small skewness and kurtosis， and the cellulose content fluctuated around 24%. The pectin and ash contents showed slight and irregular variation （Table 3）. The cellulose， hemicellulose， and lignin contents were notably higher than those of pectin and ash in the pericarps of PE10， T105， and S33205 from 12 to 24 DAP in both environments （Table 3）. For all three inbred lines in both environments， the mean contents of pericarp were all ranked hemicellulose>cellulose>lignin， except for cellulose>hemicellulose in PE10 at 12 and 14 DAP （Table 3）.

From 12 to 24 DAP， hemicellulose content in the pericarp of PE10 increased by 27.19% in spring 2014 and 27.93% in winter 2014， both of which were remarkably higher than those of T105 （15.81% and 12.63%） and S33205 （13.11% and 13.06%）. At 12 DAP， the hemicellulose content in the pericarp of the three inbred lines was ranked S33205>T105>PE10 in both environments， in accordance with the tenderness trait， suggesting that pericarp hemicellulose content affects pericarp tenderness from early kernel development.

In contrast to hemicellulose content， the lignin content in the pericarp of the three inbred lines gradually increased to a peak， before decreasing slightly to a maintenance level （Table 3）. The peak time of lignin content in three inbred lines was different in different environments—PE10 peaked at 20 DAP in both environments， T105 peaked across a two-day interval （20-22 DAP）， and S33205 peaked across a four-day interval （18-22 DAP）. The variation span in pericarp lignin content from 12 to 24 DAP was ranked PE10>T105>S33205 in spring 2014 and T105>PE10>S33205 in winter 2014. However， the variation span in lignin content from 12 DAP to peak DAP was always ranked S33205>T105>PE10 in both environments （Table 3）. This finding is consistent with the tenderness trait， suggesting that pericarp tenderness and lignin content follow a similar trend during kernel development.

Variation in the main components of the pericarp over time within inbred lines across two environments

Between environments， pericarp hemicellulose content did not significantly differ in PE10 or T105 at 12， 14， and 24 DAP （Table 3）， but significant or highly significant differences were observed at 16， 18， 20， and 22 DAP. For the average of three inbred lines， spring produced higher pericarp hemicellulose contents than winter at each sampling point. Similarly， the pericarp lignin content average in spring was higher than that in winter on each sampling day except 22 DAP， and three inbred lines had higher peaks of pericarp lignin content in spring than in winter. In contrast， the cellulose， pectin， and ash contents varied irregularly during the period of measurement （Table 3）.

Correlations between pericarp tenderness and pericarp components in super sweet corn

During kernel development， pericarp hemicellulose content had a highly significant correlation （r≥0.87; P<0.01） with pericarp tenderness （puncture reading） in both environments in all three inbred lines. Similarly， pericarp lignin content had a highly significant correlation （r≥0.71; P<0.01） with pericarp tenderness （Table 4）. However， the cellulose， pectin， and ash contents in the pericarp were not significantly related to pericarp tenderness. Similarly， pericarp tenderness at the optimum picking time had a highly significant correlation with pericarp hemicellulose （r≥0.90; P<0.01） and lignin contents （r≥0.70; P<0.01） in spring and winter 2014 in the ten inbred lines （Table 5）. Pericarp tenderness was not correlated with cellulose， pectin， or ash contents in either environment （Table 4， Table 5）.

Discussion

An unsuitable harvest date for sweet corn can decrease the nutritional and sensory value of kernels[36]. Harvesting too early could result in excess water content and insufficient dry matter in the kernel， as well as low yield， poor edible value， and inappropriate preservation， while harvesting too late could result in more starch， reduced pericarp tenderness and faded sweet fragrance. Given the practical significance of the conclusions， pericarp tenderness at the optimum picking time was considered in this study.

The data showed that pericarp tenderness （puncture reading） increased with increasing DAP， indicating that factors affecting pericarp tenderness accumulate with kernel development. The average accumulative rate of factors determines the average change rate of pericarp tenderness. According to our results， PE10 had the fastest cumulative speed in affecting factor because of the biggest mean range of pericarp tenderness from 12-24 DAP among three inbred lines. However， PE10 had the best pericarp tenderness （smallest puncture reading） at each sampling time， suggesting that it has better initial pericarp tenderness decided by itself. Thus， breeding super sweet corn with good pericarp tenderness should be based on the selection of fundamental breeding material with excellent pericarp tenderness.

As is well-known， crop performance can change with planting environment. This study identified different pericarp tenderness in inbred lines measured on the same sampling day at the two sites （Wuhan and Lingshui）. The stronger light and higher temperatures in Wuhan promoted the accumulation of factors affecting pericarp tenderness. However， the two curves for pericarp tenderness in the same inbred line in spring and winter 2014 had similar initial and terminal values （puncture readings）， as well as approximate pericarp tenderness on the optimum picking days （Fig. 2）. This demonstrates that environment changes the average accumulation rate of the factors affecting pericarp tenderness.

It was once thought that pericarp tenderness was mainly affected by pericarp thickness， as determined by the number of layers and cell size of the pericarp[37]， which were negatively correlated[4，15，38]. As a result， selecting for a thin pericarp became the main target for improving pericarp tenderness in sweet corn. However， recent results have shown that pericarp thickness might not be the main factor affecting fresh corn eating quality[16]. The degree of fibrosis and cell wall lignification likely play an important role in pericarp tenderness[39]. Our results indicate that the puncture reading was related to pericarp hemicellulose and lignin contents during kernel development， particularly on the optimum picking day， with higher puncture readings reflecting higher hemicellulose and lignin contents. In other words， high hemicellulose and lignin contents in the pericarp reduce pericarp tenderness. Pericarp tenderness is a complex trait， involving the physical structure and chemical composition of the pericarp， Studies on pericarp tenderness should incorporate both factors and their interactions.

Conclusions

Pericarp tenderness in super sweet corn gradually declines with kernel development and environments can promote or hinder the alternation rate of pericarp tenderness. Pericarp tenderness remained dependent on each inbred line. Variation in the hemicellulose content in the pericarp of the three inbred lines was similar to that for tenderness in the two environments， and while lignin content varied along a single-peak curve. Hemicellulose and lignin contents were significantly correlated with pericarp tenderness （P<0.01）. The coefficients of correlation and variation over time revealed that hemicellulose and lignin contents in the pericarp are important chemical factors affecting pericarp tenderness. However， PE10， with the best pericarp tenderness， accumulated hemicellulose in the pericarp faster than the other two inbred lines， indicating that factors other than those included in this study could affect pericarp tenderness.

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